An active matrix organic light emitting diode (AMOLED) type display due to its higher refresh rate and its shorter response time is widely used in display apparatus. Organic light emitting diode elements are configured to emit light beams in the AMOLED type display. The AMOLED includes a plurality of pixel units and a plurality of pixel driving circuits, which correspond to the pixel units respectively. The pixel driving circuit is configured to drive the brightness of a corresponding one of the pixel units, and a control circuit is configured to control the pixel driving circuits. The pixel driving circuit includes a switching transistor, a driving transistor, and a storage capacitor. The switching transistor receives a scan signal from a corresponding scan line, and turns on for loading a data signal on a corresponding data line when the scan signal is effective, such as in a high level voltage. The storage capacitor is being charged by the loaded data signal. When the switching transistor turns off, the storage capacitor discharges and the driving transistor turns on for providing a current to the OLED, thus the OLED emits light. However, driving transistors in the pixels unit of the OLED display may be subject to manufacturing variations or operating variations. Due to such variations, transistor threshold voltages between different display pixels may vary. Variations in transistor threshold voltages can cause the pixels to produce amounts of light that do not match a desired image. A method for compensating the transistor threshold voltage can solve the above-mentioned light variation problem. In this method, a detecting time period is needed for adjusting a driving voltage provided by the pixel driving circuit, based on a threshold voltage of the driving transistor, or the current passing through the OLED before displaying period. The driving voltage adjusted based on the threshold voltage of the driving transistor is different from the driving voltage adjusted based on the current provided to the OLED. Thus, there is room for improvement in the art.